In recent years, a magnetic recording medium such as a magnetic disk has been requiring increases in capacity and recording density due to remarkable technical innovation; hence an increase in precision in surface processing of various substrates has also been required.
Along with the increases in capacity and recording density of recent years, a spacing between a recording disk and a magnetic head, i.e., the floating height of the magnetic head has been becoming significantly low, and lowering the floating height of the magnetic disk causes a contact between a projection and the magnetic head hence a head crash if there is the projection on the surface of the magnetic disk, resulting in scratches on the disk surface. Also, even with a minute projection that does not cause the head crash, the contact with the magnetic head causes an error that occurs during reading and writing of information. Furthermore, the magnetic head is brought into sticking with the surface of the magnetic disk to thereby cause the trouble of not being floated.
As means for preventing the close contact between the recording disk and the magnetic head, surface processing referred to as texturing in which minute streaks are formed on the surface of the recording disk has been performed. The texturing enables a coercive force in a recording direction, i.e., the recording density of the disk to be increased by controlling the orientation of crystal growth at the time of formation of a metal magnetic layer onto the disk substrate.
As a texturing method, a method such as slurry polishing in which slurry composed of abrasive grains is attached onto the surface of an abrasive cloth and polishing is performed with the cloth, has been used. However, in the case of texturing, it can be said that waviness after the polishing should be reduced and a hard disk surface currently having an average surface roughness of 1 nm or more should be further smoothed in order to meet the increase in recording density due to the recent rapid increase in recording capacity (a target average surface roughness is 0.5 nm or less). For this reason, the attainment of fibers further made finely thinner has been desired for the abrasive cloth for polishing a hard disk surface.
However, the abrasive cloth that utilizes a current sea-island type conjugate fiber spinning technology has a limitation of a single fiber fineness of 0.01 dtex (equivalent to a diameter of 1 μm), and therefore has not had a sufficient level that can respond to the above-described needs (Patent Literature 1).
Also, a method for obtaining an ultrafine nonwoven fabric made of polymer blend fibers has been described (Patent Literature 2); however, a single fiber fineness obtained by it is 0.001 dtex (equivalent to a diameter of 0.3 μm) at the finest level, which has not also been a sufficient level that is able to respond to the above-described needs. Furthermore, an abrasive cloth with a single fiber fineness of 0.3 dtex or below using polymer blend fibers has been disclosed (Patent Literature 3), and the literature has also described that a single fiber fineness of 0.0003 dtex (equivalent to a diameter of 0.2 μm) can be obtained, which is certainly ultrafine as a single fiber fineness. However, it has also described that the obtained single fiber fineness of the ultrafine yarns described in Patent Literature 3 is decided by dispersion of an island polymer in the polymer blend fibers, and since, in a polymer blend system used in Patent Literature 3, the dispersion of the island polymer is not sufficiently carried out, a single fiber fineness of 0.0003 dtex (equivalent to a diameter of 0.2 μm) and that of 0.004 dtex (equivalent to a diameter of 0.6 μm) are mixed, resulting in a large variation of a single fiber fineness in obtained ultrafine yarns. In addition, in the case where they are used for the above-described surface abrasive cloth for a hard disk, abrasive grains cannot be uniformly carried on the abrasive cloth due to the large variation in fineness, and consequently there has arisen the problem that smoothness of the surface of the hard disk is rather reduced.
Meanwhile, as a technology for making the fibers comprising a nonwoven fabric finely thinner, an electrospinning has been attracting attention in recent years.
This is a technology in which a polymer is dissolved in an electrolyte solution and then extruded from a spinneret, and in the process of this, a high voltage of several thousands to thirty thousands is impressed to the polymer solution, and then fibers are made finely thinner by the high-speed jet of the polymer solution followed by bending and expansion of the jet. Using this technology may enable a single fiber fineness of the order of 10−5 dtex (equivalent to a single fiber diameter of several tens nm) to be provided, which is 1/100 or less in fineness, or 1/10 or less in diameter in comparison with the conventional polymer blend technology. A polymer that can be subjected to the technology is mostly a biopolymer such as collagen, or a water-soluble polymer; however, a thermoplastic polymer may be dissolved in an organic solvent to be then processed by the electrospinning. However, as has been described in the literature “Polymer, vol. 40, 4585 (1999)” (Nonpatent Literature 1), strings that are ultrafine yarn parts are connected therebetween by beads (diameter of approximately 0.5 μm) that are polymer puddle parts in many cases, and therefore there has been a large variation in single fiber fineness in a nonwoven fabric in terms of an ultrafine fiber nonwoven fabric. For this reason, an attempt to suppress generation of beads to thereby uniform a fiber diameter has been made; however, the variation has still been large (Nonpatent literature 2).
Also, a nonwoven fabric obtained by the electrospinning is provided by evaporation of a solvent in the process of fiberization, and therefore a resulting fiber aggregate is not orientationally crystallized in many cases and its strength is also lower than that of a conventional nonwoven fabric, which have been causing a large limitation to an application development. In addition, the electrospinning itself has had a big problem as a producing method, i.e., there have been problems that a size of a resulting nonwoven fabric is as small as approximately 100 cm2, and productivity is at most several grams per hour that is considerably lower than that of conventional melt spinning. There have further been problems that a high voltage is required, and an organic solvent and ultrafine yarns float in the air.
Also, as a method for producing an ultrafine fiber nonwoven fabric, a procedure utilizing cellulose fibrils has been known (Patent Literature 4). More specifically, this is a procedure in which a beating technique for pulp is applied to cuprammonium rayon to thereby fine the average diameter of fibers down to approximately 200 to 300 nm, and then the fined fibers are arranged in a mesh-like form on an ultrafine fiber nonwoven fabric made of polyester by a papermaking method.
However, the beating technique that has been conventionally established is intended for only cellulose, and it has not been possible to fine a synthetic polymer such as polyester or nylon down to its diameter of 200 to 300 nm by beating. This has been because cellulose is originally composed of microfibril aggregates whereas the synthetic polymer does not have such a clear fibril structure and therefore the beating does not cause fibrillation but pulverization. In addition, Patent Literature 4 describes a method for inducing acetic acid bacteria to produce cellulose and then configuring a structure in which cellulose nanofibers are arranged in a mesh-like form on an ultrafine fiber nonwoven fabric made of polyester. Industrial utilization of the method has been difficult, however, since the productivity of this method is too low.
Meanwhile, cellulose fibers as described above have had the problem of their essentially having poor dimensional stability due to the presence of water or moisture. Therefore nanofibers made of a synthetic polymer having good dimensional stability have been required.
Also, cellulose fibrils obtained by the conventional beating technique cannot be produced with uniform fiber diameters, which is likely to cause nonuniform pore diameters, and therefore a procedure other than cellulose fibrillation has been required.
Furthermore, also in order to control the chemical resistance, thermal resistance, affinity with a support, and the like of ultrafine fibers forming into a mesh-like structure, a method for producing the mesh-like structure composed of nanofibers made of a synthesis polymer that has a wide variety of types, instead of cellulose, has been required.
As described above, an attainment of fibers to be referred to as nanofibers, which have no limitations on the shape or polymer used in the production, can be widely applied and developed, exhibit a small variation in single fiber finenesses and have an extremely small single fiber diameter, has been required.
Also, in order to perform more precise polishing, it is required that the fibers comprising an abrasive cloth be finer in diameter and the sheet be softer; the polishing rate achieved using, such a cloth, however, is correspondingly reduced. Accordingly, in order to obtain sufficient polishing rate, a procedure in which the tension applied to the abrasive cloth during polishing is set higher thereby enhancing contact between the abrasive cloth and the polishing target has commonly be used. However, setting the tension higher causes problems such as reduced stability during processing and elongation of the abrasive cloth sheet. This in turn may lead to other problems including the occurrence of defects such as scratches on the surface of the polishing target. Therefore in order to prevent the above problems, the attainment of an abrasive cloth that can resist such higher tension has been required.    Patent Literature 1: Japanese Patent Unexamined Publication No. 2002-224945    Patent Literature 2: Japanese Patent Unexamined Publication No. Hei10-53967    Patent Literature 3: Japanese Patent Unexamined Publication No. 2002-79472    Patent Literature 4: International Published Patent Application No. 97/23266 pamphlet    Nonpatent Literature 1: Polymer, vol. 40, 4585 (1999)    Nonpatent Literature 2: Science, vol. 285, 2113 (1993)